Defined: eval function computes a string, and executes the JavaScript code
Syntax: eval (string): string is necessary to calculate the string, which contains the statement or expression to be evaluated to execute JavaScript, returns value by calculating the string
Precautions: eval covering property or eval () methods of imparting other properties, and call it the property, the ECMAScript implementation allows a throw exception EvalError
The above is the explanation of the eval function in W3School. Before, I used eval mainly to convert the json string into a js object after obtaining the data transmitted from the background to the foreground. Later, I used JSON.parse (JSON object parsing) and never touched it again. eval did not go deep into the principle and function of eval. When learning execution context yesterday , I found a different side of eval in Uncle Tom ’s blog.
Eval has a concept: call context. It uses the variable that calls it to act on the environment. If a function defines a local variable x and then calls eval("x"), it will return the value of the local variable. If you call eval("x=1"), it will To change the value of a local variable, it can also be used to declare a new local variable and declare a local function.
Eval is not inside the function. After the context is called, the variable is placed in the global context;
eval is inside the function. After the context is called, the variable is placed in the local context, and this local context will be destroyed as the function exits.